Separate texture in viewport
Assigning separate materials to a view port is not possible ( as far I know )

Alternative:
With the Node Wrangler plugin you can press Ctrl + Shift + Click on a node, and it will change it to that specific node. So you can just add an image texture to your node an swap between them.



See: http://gregzaal.github.io/node-wrangler/ ( Emission Viewer )